Detection and reduction of dielectric breakdown in semiconductor devices

ABSTRACT

Methods for detecting the breakdown potential of a semiconductor device having a thin dielectric layer are disclosed. The method includes measuring a spectroscopy of the thin dielectric layer and determining whether the spectroscopy exhibits the presence of a breakdown precursor (H 2 , H interstitial radical, H attached radical, and H attached dimer). Preferably, the method is carried out in the presence of a substantially significant applied electric field across dielectric layer. A semiconductor device tested in accordance with this method is also disclosed. Additionally, methods for reducing dielectric breakdown of a semiconductor device having a thin dielectric layer involving the substitution of a second molecule for H 2  molecules present in the dielectric. This second molecule preferably does not react with Si or O to form an undesired attached state and may be an inert gas having a molecular size approximating that of a Hydrogen atom, such as Helium. A semiconductor device made using this method is also disclosed.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims benefit under 35 U.S.C. §119(e) to provisionalpatent application Ser. No. 60/506,453 filed on Sep. 26, 2003 entitled“Mechanism of Dielectric Breakdown.”

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to methods for improving semiconductordevices and, in particular, to methods and apparatuses for the detectionand reduction of dielectric breakdown in such devices.

2. Background Art

In general, the gate oxide thickness for the current generation offield-effect transistors (FET) is approximately 50 Angstroms. The nextgeneration of FETs will require thinner oxides in order to achievedesired miniaturization. However, with current technology the electroncurrent that would leak through a thinner gate oxide during transistorusage would be too large to prove useful. In fact, such large leakagecurrents would likely lead to permanent damage of the oxide layer,potentially damaging the FET in minutes. These leakage currents arethought to be due to dielectric breakdown in the insulating layers. Thismakes dielectric breakdown a critical issue in the miniaturization ofFETs and other semiconductor devices. Yet, little, if anything, isunderstood about the mechanism responsible for dielectric breakdown inthin dielectric layers in electronic devices.

It is known the operating electric field increases as the dielectricthickness decreases. The electric field is the ratio of the appliedvoltage across the dielectric and its thickness. If the thickness isreduced by a factor of 2, then one could maintain exactly the sameelectric field by reducing the applied voltage by the same factor of 2.Unfortunately, one cannot scale the gate voltage down proportionally tothe thickness because the voltage becomes too small to control thecurrent across the FET between the source and drain. Thus, thinnerdielectrics must operate with higher electric fields.

SUMMARY OF THE DISCLOSURE

The present disclosure uses quantum mechanics and molecular dynamicsstudies to derive a mechanism for breakdown in thin dielectrics. Inparticular, the present disclosure uses the case of silicon dioxide(SiO₂) gate materials in field-effect transistors (FET) to illustratethat the significantly larger electric fields associated with thindielectrics cause H (“hydrogen interstitial radical”) that can arisefrom H₂ (“hydrogen dimer”) to react with defects and irregularities thatoccur inside the thin dielectric (insulator) and at its interface withthe semiconductor (“insulator-semiconductor interface”).

Hydrogen dimer (H₂) is a bound complex of two hydrogen atoms and isknown to be ubiquitous inside current dielectrics. H₂ was neverconsidered to be the source of dielectric breakdown. However, thepresent disclosure shows the surprising result that the reaction of H₂with the defects and irregularities in the semiconductor device causessituations that can lead to increased leakage current and breakdown.

The present disclosure also teaches methods for reducing or eliminatingthe breakdown problem. In one aspect of the invention, the H₂ is pumpedoff and replaced with helium (He), for example. In addition, the presentdisclosure also teaches methods for detecting the potential failure byin situ or ex situ monitoring of spectroscopic and other characteristicsof the sites causing breakdown, their precursors, and the presence ofthe H₂ and He.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 of the drawings is a molecular model showing an H atom in aninterstitial location (“H interstitial radical”) in SiO₂.

FIG. 2 of the drawings is molecular model showing an H atom attached(“attached radical”) to an O atom that is part of the SiO₂ structure.

FIG. 3 of the drawings is a graph comparing the computed energies of aninterstitial H in SiO₂ relative to an attached H in SiO₂ as a functionof an external electric field.

FIG. 4 of the drawings is a molecular model showing two H atoms attachedat neighboring O sites (“attached dimer”) in SiO₂.

FIG. 5 of the drawings is a graph of two different computed relativeenergies: 1.) two H interstitial “apart” states minus H₂, and 2.) theattached dimer state minus H₂ dimer. Both lines are shown as a functionof the external electric field.

FIG. 6 of the drawings is a schematic representation of the energy lossto phonons from the acceleration of the electron through attachedstates.

FIG. 7 of the drawings is a graphical representation of the computedenergies for the creation of an attached H state in a zero electricfield in the vicinity of a Si dangling bond.

FIGS. 8 a and 8 b of the drawings are a flow diagram of a preferredapproach for manufacturing a semiconductor device having a thindielectric layer and detecting the breakdown potential thereof.

FIG. 9 of the drawings is a flow diagram of a first approach to reducingthe dielectric breakdown in a semiconductor device having a thindielectric layer.

FIG. 10 of the drawings is a flow diagram of a second approach toreducing the dielectric breakdown in a semiconductor device having athin dielectric layer.

FIG. 11 of the drawings is a flow diagram of a third approach toreducing the dielectric breakdown in a semiconductor device having athin dielectric layer.

BEST MODES OF CARRYING OUT THE INVENTION

The present disclosure teaches various methods for detecting thebreakdown potential of a semiconductor device having a thin dielectriclayer and methods for reducing that dielectric breakdown potential.Although the present specification is described in terms of SiO₂field-effect transistors (FET), those working in the semiconductorprocessing industry generally understand that SiO₂ reacts much the sameway as other semiconductor materials.

The semiconductor industry, as a whole, is generally trying to decreaseoxide thicknesses to achieve increased device miniaturization. Theexistence of H₂ in dielectrics is widely known to be caused by allsemiconductor processing methods. However, prior to the presentinvention, the existence of H₂ in dielectric materials was believed bythe industry to be benign.

In semiconductor devices, as the dielectric (oxide) thickness isreduced, the size of the electric field inside the gate increasessignificantly. As a result of this significantly increased electricfield, technologically relevant applied electric fields (about 10million volts per centimeter or 10 MV/cm) lead to new chemical reactionsnot experienced previously. Some combination of this increased electricfield and these resulting chemical reactions create novel electronicstates in the dielectric that strongly increase the current flow of thedielectric and cause breakdown.

First, there is a particular state where a hydrogen atom “attaches” orweakly bonds to an oxygen atom in SiO₂ that can cause electron tunnelingand breakdown. Based on quantum mechanical and force-field studies, thisattached state was found to be 1.0 eV (electron volts) higher than thepreviously known interstitial hydrogen state in zero electric field. Inan applied electric field, this attached state can become more stablethan the interstitial state. In addition, two attached H states nearbycan be further stabilized due to their large hybridization (attacheddimer state).

Second, there are two different chemical reactions that can createthermally accessible attached H dimer states under an applied electricfield: (1) cracking H₂ in an interstitial region; and (2) cracking H₂near a Silicon dangling bond (Pb center) near the Si—SiO2 interface,H₂+Si→Si—H+ (attached H). FIG. 1 shows the well-characterized H atom inan interstitial location in a SiO₂ lattice. FIG. 2 shows a state withthe H atom “attached” to an O atom that is 1.0 eV higher in energy. Thisstate leads to an asymmetry in the Si—O bond lengths of the two Sibonded to the O attached to H. The O atom shifts 0.5 Angstroms towardsH.

FIG. 3 compares the energies of an interstitial H in SiO₂ relative to anattached H in SiO₂ as a function of an external electric field. Theseresults are for a two dimensional SiO₂ slab terminated with H atoms. Thefield is applied in the upward direction in FIGS. 1 and 2. Since thedielectric constant of SiO₂ is approximately 4, the internal field is ¼the external field value. Thus, for experimental fields from 10-20MV/cm, or 40-80 MV/cm applied fields, the attached state is lower inenergy than the apart state. Thus, the attached state becomesenergetically favorable (more stable) for external fields larger than 40MV/cm (million volts per centimeter).

In other words, as the foregoing describes, the strong electric fieldchanges the chemistry of hydrogen bonding. The strong electric fieldmakes a new quantum state accessible. The attached state has a largediffuse orbital with large amounts of charge on the nearby O and Sisites. This state will hybridize strongly with any other nearby attachedstate and lead to states in the SiO₂ bandgap with a large coupling tothe metal electrodes generally found on each side of the gate oxide. Thehybridization energy has been calculated to be 1.4 eV. This hybridized“attached dimer” state is shown in FIG. 4.

FIG. 5 compares the relative energies of the attached dimer state tointerstitial H₂ as a function of an external electric field. Forsufficiently large electric fields, the attached dimer states becomesfavorable. In particular, in the graphed example, the attached dimerstate becomes energetically favorable (more stable) for external fieldslarger than 100 MV/cm. Using the dielectric constant of 4 as before, theexperimental field is 25 MV/cm for the creation of the attached dimer.

The computed energies discussed above were calculated for the specificcase of SiO₂ in a crystalline form (alpha quartz). In reality, SiO₂,like other dielectrics, is amorphous (irregular or disordered). Thisleads to strained Si—O bonds. The required electric fields for thecreation of attached states will be reduced from the computed values foralpha-quartz for attachment to the strained bonds. Our estimates of thenecessary electric fields for attached states are therefore higher (moreconservative) than what is actually required.

Electron tunneling will occur between the attached dimers and theseelectrons can dissipate energy into the gate through phonons arisingfrom the acceleration of the electron in the applied electric field.FIG. 6 shows a schematic of energy lost to phonons from acceleration ofthe electron through attached states.

The approximate observed reversibility of breakdown can be understoodfrom electric field induced chemistry. For thicker dielectrics, theelectric field is never large enough to make the attached stateaccessible and hence there is no breakdown. For thinner dielectrics,however, the electric field is large enough to make the attached statechemistry accessible and easy electron tunneling can occur. This willover time lead to permanent damage to the dielectric structure due tothe release of phonons in the gate by the mechanism shown in FIG. 6. Ifthe field is turned off, then the attached states are lost and thesystem reverts back to its original state less whatever permanent damageoccurred. Thus, one can understand the difference between soft and hardbreakdown phenomena.

FIG. 7 is another computed H₂ cracking pathway. FIG. 7 shows that H₂cracking to passivate a Si dangling bond at the Si—SiO₂ interface alongwith the creation of an attached H state is energetically favorable evenin zero electric field. Such Si dangling bonds are known to occur nearthe dielectric-semiconductor interface. This shows the formation ofattached states occurs even without an electric field near theinterface.

Based on this observed chemistry and novel electric field inducedstates, methods may be derived to detect and reduce this dielectricbreakdown mechanism. In one embodiment, the detection of the potentialor imminent failure may be performed by in situ or ex situ monitoring ofspectroscopic and other characteristics of the sites causing breakdown,their precursors, and the presence of the H₂ and He.

FIGS. 8 a and 8 b of the drawings are a flow diagram of a preferredapproach to manufacturing a semiconductor device having a thindielectric layer and detecting the breakdown potential thereof. In step802, a semiconductor device, such as a FET, having a thin dielectric isfabricated. This fabrication may be any of the presently knowntechniques as well as presently unknown techniques that may be devisedin the future. In one approach, a statistically representative sample ofthe semiconductor devices fabricated in a batch in step 802 are selectedfor testing, step 804. In another approach, all of the fabricateddevices may be desired for testing, thus step 804 would be omitted inthose instances.

In step 806, a substantially significant electric field may then beapplied to a selected area of the thin dielectric layer of eachindividual semiconductor device selected for testing. In one embodiment,the location of the selected area may be chosen based upon the locationof any leakage current in the semiconductor device. A substantiallysignificant electric field is an electric field (measured in MV *cm⁻¹ ormillion volts per centimeter) that would cause H₂ in an SiO₂ layer of asemiconductor device to react with the defects and irregularities inSiO₂. In certain semiconductor devices that have already experiencedbreakdown damage, it may be possible to detect the breakdown precursorswithout application of the significant electric field. Consequently, itmay be desirable to test the semiconductor devices without applying theelectric field.

In step 808, the spectroscopy of the one selected area of the thindielectric layer is measured under the application of the substantiallysignificant electric field. In step 810, it is then determined whetherthe measured spectroscopy exhibits—based on the criteria taughtabove—the presence of at least one breakdown precursor, i.e., H₂, Hinterstitial radical, H attached radical, and H attached dimer.

In step 812, the intensity of the substantially significant electricfield optionally applied in step 806 may be increased. In such a case,the spectroscopy of the area may be measured again (step 814) andanalyzed to determine whether this spectroscopy exhibits the presence ofat least one breakdown precursor from the group consisting of H₂, Hinterstitial radical, H attached radical, and H attached dimer (step816). In step 818, the electric field strength may be increased again toincrease the potential for breakdown chemical reactions to occur in thethin dielectric layer. If such increased testing is desired, the methodreturns to step 812. If not, then the method is concluded. Suchincreased field strength may be desirable where a semiconductor devicehas not exhibited the potential for breakdown chemical reactions underthe lower field strengths or alternatively to conduct acceleratedtesting of the device.

Measuring the spectroscopy of the selected area of the thin dielectriclayer may be performed using various methods. Based on the foregoingequations, detecting and monitoring changes in H₂ such as those due tothe creation of attached H states using Raman and infra-red spectroscopyto observe changes with and without electric field stress can be usedduring device fabrication to determine device quality and also to screenpotentially bad devices. Electron-Spin Resonance (ESR) may also be usedfor ESR active configurations (eg., the attached H state) and x-rayabsorption/scattering (XANES and XAFS) can detect local structuralchanges in the material.

Electron Spin Resonance Spectroscopy (ESR) may be used to detect Hradicals; attached H; attached H dimer; and Si E and E′-centers. TheHydrogen radical is ESR active due to its single unpaired electron. (H₂is not ESR active because its two electrons are paired in a spinsinglet.) Thus, the cracking of H₂ into two H radicals can be observedby detecting the appearance and change in the H radical spin resonanceintensity.

When the H radical attaches to an O (oxygen) in SiO₂, the unpairedelectron remains unpaired, but goes into a different ESR active state.There is a detectable g-factor (resonance energy) difference between Hradical and attached H to O. The decrease of one ESR signal along withthe associated increase in the other signal can be used to determine therate of attaching reactions or H separation reactions.

The attached H dimer state is spin singlet and is not ESR active. Thus,monitoring changes in the H interstitial radical, attached H radical ESRsignals along with their intensity changes leads to information on therate of formation of attached H dimer states. Detecting Si dangling bondESR and its changes can be used to examine the formation of the above Hstates in the interface region where the strain energy from joining theoxide and semiconductor materials together makes the formation of theabove H states more favorable.

Raman and infra-red (IR) spectroscopy may also be used to detect H₂;attached H radical and dimer; and Si—O bond vibrations. Changes in theRaman and IR signals for H₂ vibrational modes allows the observation ofthe amount of H₂ along with its cracking to form H radical. Attached Hradical and dimer will have distinct detectable vibrational frequenciesand intensities that can be monitored.

Optical absorption, reflection, and transmission may be used to detectthe attached H radical and dimer states within the SiO₂ bandgap(approximately 9 eV). These states can be detected and monitored byoptical absorption, reflection, and transmission arising from exciting abound attached electron into the conduction band.

Finally, X-ray Absorption Near Edge Structure (XANES) and X-rayAbsorption Fine Structure (XAFS) may be used to measure small structuralchanges arising from the formation of the above H attached and radicalstates. In this case, the O atom that H attaches to changes position byapproximately 0.5 Angstroms.

FIG. 9-11 illustrate three potential embodiments for reducing thedielectric breakdown of a semiconductor device having a thin layerdielectric. In general, reducing the dielectric breakdown isaccomplished by substituting H₂ molecules in the interstitial sites ofthe SiO₂ with elements or molecules that do not react with Si or O toform the undesired attached states as described above. For instance, inone embodiment, the H₂ may be reduced—in turn reducing dielectricbreakdown—by pumping it off and replacing it with helium (He) or anyother relatively inert gas that has molecular size approximating that ofthe H atom.

FIG. 9 illustrates a first method for reducing dielectric breakdown.Generally, H interstitial radicals can move easily form one SiO₂interstitial void to another, while H₂ cannot. In step 902, ultraviolet(UV) light may be applied to a selected area of the thin dielectriclayer (for example, oxide gate) in order to break the H₂ molecules intoH interstitial radicals. In step 904, pressurized Helium gas may then beapplied to the selected area to essentially push the H atoms out of thedielectric while the Helium atoms fill the interstitial region.

In a second embodiment illustrated in FIG. 10, a substantiallysignificant electric field, rather than UV light, may be applied to theselected area in order to break the H₂ molecules into H interstitialradicals (step 1002). Once again, pressurized Helium gas may then beapplied to the selected area to essentially push the H atoms out of thedielectric while the Helium atoms fill the interstitial region (step1004). The applied electric field may also be a pulsed electric field.The length of each pulse may then be adjusted such that the attachedradical state can be formed while driving the H interstitial radicalsout of the oxide with the pressurized He.

In yet another embodiment illustrated in FIG. 11, the selected area maybe grown using known techniques for growing semiconductor layers (step1102). In step 1104, a pressurized Helium gas may then be flowed intothe growing chamber in order to displace the H₂ molecules and replacethem with He during the growth process.

While various embodiments of the application have been described, itwill be apparent to those of ordinary skill in the art that many moreembodiments and implementations are possible that are within the scopeof this invention. Accordingly, the invention is not to be restrictedexcept in light of the attached claims and their equivalents.

1. A method of detecting the breakdown potential of a semiconductor device having a thin dielectric layer, the method comprising: measuring a spectroscopy of at least one selected area of the thin dielectric layer; and determining whether the spectroscopy of the at least one selected area of the thin dielectric layer exhibits the presence of at least one breakdown precursor from the group consisting of H₂, H interstitial radical, H attached radical, and H attached dimer.
 2. The method according to claim 1 further comprising applying a substantially significant applied electric field across the at least one selected area of the thin dielectric layer while measuring the spectroscopy of the at least one selected area of the thin dielectric layer.
 3. The method according to claim 2 further comprising: increasing the substantially significant applied electric field across the at least one selected area of the thin dielectric layer by a predetermined amount after measuring the spectroscopy of the at least one selected area o the thin dielectric layer; and determining whether the spectroscopy of the at least one selected area of the thin dielectric layer under the influence of the increased applied electric field exhibits the presence of at least one breakdown precursor from the group consisting of H2, H interstitial radical, H attached radical, and H attached dimer.
 4. The method according to claim 2 further comprising: repetitively increasing the substantially significant applied electric field across the at least one selected area of the thin dielectric layer by the predetermined amount after measuring the spectroscopy of the at least one selected area of the thin dielectric layer; and for each repetitive increase of the substantially significant applied electric field, determining whether the spectroscopy of the at least one selected area of the thin dielectric layer under the influence of the increased applied electric field exhibits the presence of at least one breakdown precursor from the group consisting of H₂, H interstitial radical, H attached radical, and H attached dimer.
 5. The method according to claim 2 further comprising selecting the at least one selected area of the thin dielectric layer for measurement based upon the location of a leakage current in the semiconductor device.
 6. The method according to claim 2 wherein measuring the spectroscopy includes the step of using Electron Spin Resonance Spectroscopy to measure the physical characteristics caused by the at least one selected from the group consisting primarily of Hydrogen ESR, Attached H ESR, Attached H dimer ESR and Si E and E′-center ESR.
 7. The method of claim 6 wherein measuring the spectroscopy further includes the step of using infra-red spectroscopy to measure the physical characteristics caused by the at least one selected from the group consisting primarily of H₂, attached H radical and dimer, and Si—O bond vibrations.
 8. The method according to claim 7 wherein the infra-red spectroscopy is performed with a Raman Spectroscope.
 9. The method according to claim 2 wherein measuring the spectroscopy includes the step of using Glancing Incidence X-Ray Reflection/Refraction (GIXR) to measure the physical characteristics caused by small spatial density changes in the interface (Metal/SiO2) region caused by the formation of the above H attached and radical states.
 10. The method according to claim 2 wherein measuring the spectroscopy includes the step of using optical absorption, reflection, and transmission to measure the physical characteristics caused by the group consisting of attached H radical and dimer states.
 11. The method according to claim 2 wherein measuring the spectroscopy includes the step of using X-ray Absorption Near Edge Structure (XANES) to measure small structural changes arising from the formation of the above H attached and radical states.
 12. The method according to claim 2 wherein measuring the spectroscopy includes the step of using X-ray Absorption Fine Structure (XAFS) to measure small structural changes arising from the formation of the above H attached and radical states.
 13. A semiconductor device tested in accordance with the method of claim
 1. 14. A semiconductor device tested in accordance with the method of claim
 2. 15. A semiconductor device manufactured as part of a batch wherein a statistically representative sample from the batch is tested in accordance with the method of claim
 1. 16. A semiconductor device manufactured as part of a batch wherein a statistically representative sample from the batch is tested in accordance with the method of claim
 2. 17. A method for reducing dielectric breakdown of a semiconductor device having a thin dielectric layer, the method comprising: substituting an H2 molecule in at least one selected area of the thin dielectric area with a second molecule.
 18. The method of claim 17 wherein the second molecule does not react with Si or O to form an undesired attached state.
 19. The method of claim 17 wherein the second molecule is an inert gas having a molecular size approximating that of a Hydrogen atom.
 20. The method of claim 17 wherein the second molecule is helium.
 21. The method of claim 17 wherein substituting includes growing the at least one selected are in the presence of pressurized flowing He.
 22. The method of claim 17 wherein substituting includes: applying ultra-violet light to the at least one selected area; and applying a pressurized He gas to the at least one selected area.
 23. The method of claim 17 wherein substituting includes: applying a substantially significant electric field to the at least one selected area; and applying a pressurized He gas to the at least one selected area.
 24. The method of claim 23 wherein the substantially significant electric field is a pulsed electric field.
 25. A semiconductor device made using the method of claim
 17. 26. A semiconductor device made using the method of claim
 18. 